Spatialization of denitrification by river corridors in regional-scale watersheds: Case study of the Seine river basin

Numerous studies have shown that riparian wetlands are able to reduce nitrate concentration both by plant uptake and by denitrification. Although this function of nitrate removal by riparian wetlands is well established, it remains difficult to quantify, especially at the catchment scale. This paper discusses the nitrate budget approach used to estimate nitrate retention in the riparian zones and related streams. The method is applied in sub-catchments of the Seine basin using data sets available at this scale. As a result, retention rates were computed in 174 embedded catchments, each catchment being defined downstream by a well monitored measurement station of riverine nitrate concentration. Catchments receiving more than the equivalent of 20 hab/km2 in point source pollution were excluded. The mean of retention rates computed over these 174 catchments is equal to 35%, a value that is in the range of retention rates previously estimated in the Seine river basin. Interpolations of nitrate fluxes are an important source of retention rate estimate uncertainty. To assess the significance level of our results, we changed the input parameters by ±10%. These 10% variations result in an uncertainty range of ±18% of the original rate. The statistical analysis conducted on retention rate confirmed the importance of land cover of the river corridor in the denitrification process, which is favoured by the presence of water and forest. The spatial distribution of retention rates shows a reduction of the retention variability in the most downstream catchments, which is likely linked to the nesting of the catchments. This calls for the separation of nested catchments into independent zones, which should also increase the signal to noise ratio in the variability of the retention rates estimates.